The present invention relates to knife assemblies for disk-type chippers, and in particular to a knife assembly that can employ reversible blades.
Chippers of this type are well known, and examples are found in U.S. Pat. No. 4,784,337 to Nettles et al. and in U.S. Pat. No. 4,887,772 to Robinson et al. Those chippers are of the well-known Carthage-Norman design in which a vertical disk rotates upon a horizontal drive shaft. The disk is covered by a housing with a removable hood and a spout or feeder projects from the housing for guiding logs into contact with the rotating disk. The logs are supplied, generally end-on, to a series of radially disposed knives carried on the disk. The knives are clamped in place at cutting stations around the disk, and chip slots or passages through the disk are situated at the cutting stations adjacent the knife blades. The wood chips cut from the logs move through the passages and are ejected from the machine via a casing discharge area. The spout or feeder has a stationary bedknife that cooperates with the rotary knives so that the logs are cut into uniform chips suitable to be processed, e.g. in the digester of a wood pulping machine.
The blades of the knife assemblies are the parts most subject to wear. For this reason good blade design principles should include ease of replacement and relatively low replacement cost, together with means to extend the blade wear life. When the blades become worn they are resharpened by grinding off material, and then are replaced and adjustments made to achieve proper positioning. The blades must be secured and firmly supported in the knife holder.
Reversible knives have been employed as a step in reducing machine down time and knife cost. A number of reversible blade arrangements have been proposed, and two of these are found in U.S. Pat. No. 4,669,516 to Carpenter et al. and in U.S. Pat. No. 4,047,670 to Svensson. Carpenter employs a V-shaped cross-section knife while Svensson employs a Z-shaped cross-section knife. These each have angularly disposed cutting surfaces that make the blade extremely difficult to grip and support during assembly. The knives are clamped down by a clamp bar or hold-down plate, these being rather precisely machined so that their profiles exactly match those of the knife blades that they support. Due to the odd configuration, the blades are expensive to manufacture and can be rather difficult to sharpen. The center waist or bend in the knife blade constitutes a weak sector, but is also near the place of maximum mechanical stress. Thus, these blades are subject to an accelerated rate of failure under load. Consequently both the initial cost of a chipper using these reversible knives and their cost of maintenance and operation remain relatively high.
Holmberg et al. U.S. Pat. No. 4,694,995 employs a knife cassette with a Z-cross section blade clamped down by a profiled knife holder.
Disposable knife blade assemblies have also been proposed in which the conventional resharpenable chipper knife is replaced with an adapter or cassette knife assembly. This can be comprised of a two-part knife blade, a cassette, and a hold-down top plate. One such assembly is described in U.S. Pat. No. 4,784,337. In that assembly a reversible, double-edge knife blade fits an elongated cassette that has a blade-receiving recess. In that case the blade has a keyway or cutout on its underside that mates with a rib or key on the cassette. The knife is held in place with a retaining bar, so that one edge is exposed and the other edge is covered. Adjustment of blade position is achieved by babbitting at the back surface of the cassette. Here, the cutouts or keyways can have a weakening effect, and because they must be machined rather precisely, they raise the manufacturing cost of the knife blades. However, with that system it is not possible to employ a flat blade.
In U.S. Pat. No. 4,878,772, a cassette type knife is employed, where a reversible blade is supported in a cassette which is a stepped member with surfaces supporting a flat side and one sloping side of the knife blade. The entire cassette can be fastened in place in the blade holder of the chipper disk. Generally, the cassette and blade have to be babbitted to adjust to the correct cutting depth.